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| Home > Publications database > Abdichtungseigenschaften pelitischer Gesteine: experimentelle Charakterisierung und geologische Relevanz |
| Book/Report | FZJ-2019-04539 |
1998
Forschungszentrum Jülich, Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/22637
Report No.: Juel-3596
Abstract: This work („Sealing efficiency of pelitic rocks- experimental characterisation and geological relevante") was carried out at the Institut für Erdöl und Organische Geochemie (Forschungszentrum Jülich GmbH) in the context of the Geoscientific Co-operation Phase II (Hydrocarbon Migration and Trapping), the DGMK Research Project 483 (Gas Sealing Efficiency of Cap Rocks) and and in co-operation with the Shell AG (Gas Sealing Efficiency of Sedimentary Rocks). In this work the transport properties and the sealing efficiency (molecular transport (diffusion), capillary entry pressure and single phase permeability) of selected caprocks were investigated under simulated insitu pressure and temperature conditions (up to 45 MPa effective stress and 10 MPa pore pressure, 50-200 °C). Furthermore the samples were characterised by mineralogical, geochemical and petrophysical (specific surface area and mercury porosimetry) analysis. The samples used in this study were Jurassic shales and siltstones from the Haltenbanken area offshore Norway, red silty claystones from Carboniferous and Permian intervals of Northern Germany and fine grained sandstones of a damage zone. Porosity and pore size distribution are the fundamental petrophysical parameters controlling the capillary sealing efficiency. In this work it is shown, that omnidirectional mercury intrusion analysis is unreliable for the samples under investigation because of the uncertainties relating to the correction of the surface roughness, the intrusion rate of the mercury and the existente of a micropore volume below the detection limit. For this reason a new experimental method was developed to determine the unidirectional displacement of water by a non-wetting phase (methane) and the two-phase permeability of mudrocks with low porosity and permeability. The permeability of the mudrock samples was investigated as a function of increasing effective stress. The initial permeability is reduced by one to two orders of magnitude with increasing effective stress. The measured permeabilities at $\sigma_{eff}$ = 45 MPa are generally lower than 5 $\cdot$ 10$^{-21}$ m$^{2}$ (5 nDarcy). Attempts to find correlations between permeability and the petrophysical parameters were unsuccessful. The validity of the widely used Kozeny-Carman equation was tested. This relation predicts a drastic permeability decrease for porosities below 1%, which does not correspond to the experimental results. Isotope fractionation of methane and ethane by diffusive transport was measured an water-saturated rocksamples at elevated pore pressure and temperature conditions (3 MPa, 90 °C). The results show a significant change in the isotopic composition of the diffused gases with time. The calculated effective diffusion coefficients are lower for the Keavier isotope species. The isotope fractionation effect is higher for methane than for ethane. For the First time effective diffusion coefficients for molecular nitrogen and the stress dependence of diffusive transport (methane and nitrogen) has been investigated. The results show, that the effective diffusion coefficient and the diffusive transport (based an mass units) is generally higher for nitrogen than for methane. Organic matter significantly reduces the effective diffusion coefficients. This effect is stronger for methane than for nitrogen and decreases with increasing temperture. With increasing effective stress (up to 40 MPa) the diffusion coefficients are reduced by 50% and the molecular flux by 66% respectively. No correlation was found between diffusion parameters and the deformation bands in the fine-grained sandstones. Based an a simplified nadel, it is shown how the experimental data can be applied to a geological System. These calculations show, that the mudrocks under investigation can be classified as membrane seals. Pressure driven Darcy-flow, occuring after the capillary entry pressure (seal failure) has been exceeded, is by up to six orders of magnitude higher than calculated diffusive transport under the same conditions. The effect of diffusive losses an a commercial size hydrocarbon accumulation, in most cases, negligible. The differentes in the effective diffusion coefficients for the $^{13}$C- and $^{12}$C-hydrocarbon gases under investigation indicate the possibility of a strong effect of diffusive transport an the isotope composition of gaseous hydrocarbon accumulations.
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